it was said you could think of the gems as a reputation rewards from the black prince. you can only ever have one equipped in your current gear (unique-equipped), but you can have as many as you want in your inventory. if it would be so easy to get you wouldnt by them for sigils :)

he said it in a tweet a week or 2 back, also from simple deduction, one can assume that the fect we are gathering moret han the 10 required that should players need to aquire a second or third gem from wrathion when all quest objectives have been met, that it requires a further set of 10 sigils of each type per gem.